The merozoite interacts in a receptor specific manner with the erythrocyte surface and is the stage against which immunity may work to block invasion. Thus, merozoite surface components are of interest for their role in erythrocyte recognition and as antigens for induction of protective immunity. We are identifying P. knowlesi receptors for attachment to monkey and human erythrocytes. We are studying one antigen on the merozoite surface that undergoes antigenic variation to understand the molecular basis for this variation. We are studying P. falciparum mutants that use an alternative receptor for invasions of human red cells. We are attempting to adapt P. falciparum to subprimates as an animal model for vaccine trials.